1. Field of the Invention
The invention relates to a medical implantable lead of the type adapted to be implanted into a human or animal body for monitoring and/or controlling of an organ inside the body, and having a fixation means in a distal end, adapted to penetrate into the tissue of the organ to fixate the lead to the organ, and an electrode member to receive and/or transmit electrical signals from or to the organ.
The invention also relates to a method for connecting a medical implantable lead to an organ inside a human or animal body for monitoring and/or controlling the organ by receiving and/or transmitting electrical signals from or to the organ.
2. Description of the Prior Art
It is common knowledge to attach a distal end of a medical implantable lead to an organ inside a human or animal body, in order to monitor and/or control the organ by means of electrical signals. One example of such an application is connection of a pacemaker and/or a defibrillator to a heart but also other applications and other organs could be conceivable. Accordingly, everywhere in the following description and claims where reference is made to a heart, it is to be understood that it shall also apply, where relevant, also to other organs.
Two main methods for connecting the lead to the organ can be distinguished. For so-called passive fixation, the tip of the lead is provided with tines, fins or the like that are adapted to engage in the trabecular network inside the heart. After a few days the lead will be further anchored to the heart by being overgrown by tissue. The other connecting method is a so called active fixation, by which the tip of the lead is provided with a sharp fixation means, such as a rotatable helix or a needle provided with barbs, which is adapted to penetrate into the heart wall and engage thereto. An active fixation lead has some advantages over a passive fixation lead in that the fixation will be more distinct and secure, at least during the first critical days after implantation, and allows as a rule attachment in any arbitrary desired position.
Also the electrode in the distal end of the lead can be provided in essentially two main ways. One possibility is a contact electrode, by which the electrode has an electrode surface which abuts the surface of the tissue. Another is a penetrating electrode, by which the electrode is penetrated into and located in the tissue.
A passive fixation lead is always combined with a contact electrode. An active fixation lead has its electrode normally integrated with the active fixation means such that the helix or the needle also functions as the electrode. However, it is also known active fixation leads, where the fixation means, e.g. a helix, is electrically insulated and instead the lead is provided with a separate contact electrode, which abuts against the surface of the tissue.
When implanting an electronic device in a body, it is important to ensure a long operational life in order to avoid frequent replacement of the device. Normally, it is the battery life in the electronic device that is the limiting factor. Accordingly, it is important to keep the energy consumption of the device down to attain a long operational life. It is primarily two parameters which are essential for a low energy consumption. That is the impedance of the electrode and the capture threshold of the signal transfer between the electrode and tissue. With a high impedance in the electrode, the current consumption can be restricted and with a low capture threshold, the required voltage for transferring signals to the tissue will be low.
A high impedance is normally achieved by reducing the electrode area. In the case of a helix electrode, which is the most common example of a lead having an active fixation, a high impedance can be achieved by partly providing the helix with an electrically insulating coating. Low capture threshold can be achieved by enabling good contact of the electrode with excitable tissue. When using an active fixation lead with the electrode integrated in the fixation element, e.g. a helix which is screwed into the tissue, a good electrical contact is normally attained initially. However, since the electrode is causing a trauma as it penetrates into the tissue, the electrical contact will deteriorate within a few days due to the inflammatory process and the subsequent fibrous capsule formation.
As mentioned it is known to use active fixation leads that have a separate contact electrode, e.g. by combining an insulated helix with an electrode surface provided on the tip of the lead, which abuts against the surface of the tissue when the helix is screwed into the tissue. However, in this case it is not ensured that the electrical contact will be good in case the helix is not inserted deep enough into the tissue or the helix is partly dislodged.